This invention is concerned with optical fiber gyroscopes.
A mechanical gyroscope utilizes the inertia of a spinning mass to provide a reference direction useful in various applications, such as the navigation of an airplane or a spacecraft. The moving parts required in a mechanical gyroscope, however, cause some undesirable attributes, such as high drift rates resulting from friction. The ring laser gyroscope was developed to avoid some of these difficulties.
The ring laser gyroscope maintains a constant frame of reference by circulating massless light waves in a closed path. A typical ring laser gyroscope, for example, consists of a triangular resonant cavity defined by three corner mirrors. A gas laser generates a monochromatic light beam which is split into two beams. These beams are made to propagate in clockwise and counterclockwise directions in the cavity. If the gyroscope is rotated about an axis which has a component normal to the plane of the optical path, the frequency of one of the beams will be increased, while the frequency of the other beam will decrease, because of the Doppler effect. The beams can then be extracted and combined to produce a beat frequency which is indicative of the magnitude and direction of the rotation.
Although ring laser gyroscopes have been developed to the point of production, limitations in this device have motivated researchers to seek other design approaches. One of the most significant difficulties with ring laser gyroscopes involves the cavity mirrors. Although the laser beams within the optical cavity would ideally be totally reflected by the mirrors, the reflective surfaces cannot be made perfectly reflective. Consequently, a small amount of light is scattered backward from microscopic scattering centers in each surface, thereby transferring energy to the oppositely travelling wave. At slow rates of rotation, this coupling causes the frequencies of the two beams to lock together at a single frequency, thereby preventing the measurement of such rates.
Although various solutions have been implemented to compensate for this lockin problem, one appealing possibility is the use of an optical fiber to serve as the optical path for the gyroscope. In addition to eliminating mirrors, such an optical fiber gyroscope offers the advantages of low cost and compact size in an apparatus with no moving parts. In its simplest form, a fiber optic gyroscope includes a laser whose output is directed toward a beam splitter, which divides the laser beam into two equal parts. Each beam is coupled into an end of a coiled optical fiber so that one beam traverses the coil in a clockwise direction while the other beam propagates in the counterclockwise direction. Upon exiting the ends of the fiber coil, the beams are recombined in the beam splitter and form an interference pattern at the output of the gyroscope.
The reciprocal phase shifts introduced into the counterpropagating beams by linear propagation will cancel at the output, but rotation of the coil will cause nonreciprocal phase shifts which add together and can thus be used to indicate the magnitude and direction of rotation.
Although the problem of lockin is eliminated in the optical fiber design, other problems arise because this gyroscope will also detect any nonreciprocal effect which causes a phase shift, such as the Faraday effect, the nonlinear Kerr effect, and polarization mode coupling. Polarization mode coupling results from the need to preserve the state of polarization of the laser radiation inside the fiber.
Polarization mode coupling occurs because a single mode fiber actually supports two polarization modes which have similar transverse mode structures but are polarized at 90.degree. with respect to each other. In an ordinary single mode fiber, slight ellipticities in the core cause these polarization modes to have slightly different propagation constants. Coupling between the polarization modes can be caused by many physical mechanisms, such as bending, strain, fiber imperfections, pressure, and thermal or acoustic fluctuations. As a consequence, optical fiber gyroscopes in the prior art have required the use of a perfect single mode birefringent fiber, which will preserve the state of polarization of light passing through it, in order to achieve high sensitivity. It would be much more advantageous, however, to provide an optical fiber gyroscope which could exhibit a high degree of sensitivity while using various kinds of optical fibers, including multimode circular fibers.